I built a unipolar stepper motor driver circuit using 4 IRLZ44N MOSFETS to switch the 12vdc to the motor windings. The circuit is working fine, but I just realized that I didn't include any counter-emf diodes on the outputs. Should I add them?

I'm missing something obvious-
The diode in a MOSFET (really a transistor with a shorted junction) will not conduct any current from drain to source because of its orientation except in reverse breakdown.

When an inductor is switched off, current will continue to try and flow towards the drain and the back biased cathode of the body diode. How is back emf being suppressed? You cannot push current back through the diode's cathode.

Figure 8 in AN-936 shows a zener across D-S. That makes sense and works well.

When an inductor is switched off, current will continue to try and flow towards the drain and the back biased cathode of the body diode. How is back emf being suppressed? You cannot push current back through the diode's cathode.

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In a H-bridge with an inductor between the legs they conduct, yes. Let's say the upper left and the lower right switch is ON. On the inductor you have + on the left and - on the right. Turn off the switches , the inductor generates an inverted voltage that can be higher than the DC bus. Now all switches are off but since you'd have a (big) positive voltage on the right of the inductor and negative on the left the lower left mosfet diode and upper right mosfet diode will conduct. They will be forward biased.

I forgot the H bridge part. I should of realized that when he said 4 MOSFET's. Kinda obvious.

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I don't think it's an H-bridge; it's four individual MOSFETs, each on a separate microcontroller output, and each controlling a separate winding of the stepper. I would post a schematic, but I don't have one since I sort of based the design on previous projects I have done.

I don't think it's an H-bridge; it's four individual MOSFETs, each on a separate microcontroller output, and each controlling a separate winding of the stepper. I would post a schematic, but I don't have one since I sort of based the design on previous projects I have done.

I apologize for the confusion.

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Well in this case I think, yes, you should add diodes on the windings, since the mosfet body diode will never conduct. (I assume something like positive voltage --> motor winding --> drain of an N-FET --> source goes to GND) diode on the motor winding pointing to positive.